1. Field of the Invention
The present invention relates to a valve lifter made of titanium alloy and a method of manufacturing the same.
2. Description of the Related Art
A valve lifter in a valve operating mechanism of an internal combustion engine for racing is generally made of titanium. In car races there are few demands in terms of cost, and therefore the valve lifter is subjected to a surface treatment such as expensive ion plating in order to improve wear resistance. On the other hand, as for applying the titanium valve lifter to mass-produced vehicles, there are no practical cases of using a surface-treated titanium alloy especially due to the problem of costs because titanium itself is expensive and requires an expensive surface treatment. In addition, the valve lifter for mass-produced vehicles requires better properties in terms of the wear resistance than that for racing vehicles. A known example of the valve lifter made of titanium alloy is a valve lifter in which a surface part of the body thereof is hardened so as to form an oxygen diffusion layer. This valve lifter includes an adjusting shim in a sliding surface of the valve lifter body on which a cam slides. The sliding surface particularly requires wear resistance, and the adjusting shim is made of a material particularly excellent in sliding properties, for example, such as hard metal including carbon steel, stainless steel, and the like (refer, for example, to JP 7-139314 A, pages 2 to 3 and FIG. 1).
The above-mentioned document describes a valve lifter 01 made of a titanium alloy as shown in FIG. 6. This valve lifter has a structure in which a surface part of the body thereof is hardened so as to form an oxygen diffusion layer 02 and an adjusting shim 05 made of a hard metal such as a carbon steel or a stainless steel is provided in a sliding surface 04 as an upper surface part 03 of the body of the valve lifter 01. The sliding surface 04 requires increased wear resistance because of sliding contact with a valve operating cam and is made of a hard metal which is particularly excellent in wear resistance and sliding properties, for example, such as carbon steel or stainless steel.
This valve lifter made of a titanium alloy includes the adjusting shim made of a hard metal such as a carbon steel or a stainless steel, which is a material excellent in wear resistance and sliding properties, to form the sliding surface of the top surface part of the body on which the cam slides. Accordingly, this adjusting shim increases the weight of the top part of the valve lifter. The increase in weight of the top part of the valve lifter causes an increase in the inertia weight of the valve lifter, thus reducing the effect of the titanium valve lifter employed for weight reduction. Furthermore, each tappet clearance is adjusted by the heavy adjusting shim, and valves have considerable variation in inertia weight. This can increase operating noise of the valve operating mechanism.
To solve the aforementioned problem, an inner shim type valve lifter has been proposed in which the adjusting shim is disposed between an inner surface of the valve lifter and the top end of a valve stem, and the body of the valve lifter and the sliding surface are integrally formed. In the valve lifter according to this proposal, the oxygen diffusion layer needs to be thicker on the sliding surface of the valve lifter on which the cam slides, than in the other parts of the valve lifter. However, there is no literature that describes on a concrete specification thereof. Moreover, the oxide layer could be separated during the sliding. Consideration has therefore been made for processes to grind valve lifters one by one and to remove a part of the oxide layer as the outermost surface part by shot blasting, but greatly increased manufacturing costs have been a problem in particular.
Under the circumstances as described above, it is the main object of the present invention to provide a lightweight and high-strength valve lifter which is made of a titanium alloy and has an excellent wear resistance and sliding properties, particularly, good properties in sliding on the cam. The present invention also provides a method of manufacturing the same.